Line pressure regulating system for automatic power transmission

ABSTRACT

A system for regulating a line pressure in a hydraulic control circuit for an automatic power transmission that is capable of reducing the line pressure to a sufficiently low level to prevent shift shock at the time of gear shifting. When gear shifting is to be effected, a gear shift command signal is supplied to an electronic line pressure control circuit including a monostable multivibrator to keep the multivibrator in a metastable state for a predetermined period of time. During the time period the electronic line pressure control circuit generates a line pressure reduction signal which is supplied to a pressure regulating valve arrangement, thereby reducing the line pressure to the sufficiently low level. The line pressure is normally maintained at a sufficiently high level to prevent slippage of the friction-drive-establishing devices of the automatic power transmission.

United States Patent 1191 11 11 3,710,651 Marumo et a]. 1 Jan. 16, 1973s41- LINE PRESSURE REGULATING 3,561,296 2 1971 lijima ..74/869 SYSTEMFOR AUTOMATIC POWER 3,583,259 6/1971 Shimosaki ..74/864 TRANSMISSIONPrimary Examiner-Arthur T. McKeon Attorney-John Lezdey [57] ABSTRACT Asystem for regulating a line pressure in a hydraulic control circuit foran automatic power transmission that is capable of reducing the linepressure to a sufficiently low level to prevent shift shock at the timeof gear shifting. When gear shifting is to be effected, a gear shiftcommand signal is supplied to an electronic line pressure controlcircuit including a monostable multivibrator to keep the multivibratorin a metastable state for a predetermined period of time. During thetime period the electronic line pressure control circuit generates aline pressure reduction signal which is supplied to a pressureregulating valve arrangement, thereby reducing the line pressure to thesufficiently low level. The line pressure is normally maintained at asufficiently high level to prevent slippage of thefriction-drive-establishing devices of the automatic power transmission.

5 Claims, 4 Drawing Figures [75] Inventors: Nagayuki Marumo; Namio Irie,both of Yokohama, Japan [73] Assigneez lflissan Motor Company, Limited,Yokohama, Japan [22] Filed: March 8, 1971 [21] Appl. No.: 121,776

[30] Foreign Application Priority Data I March 18, 1970 Japan 45/22 483521 u.s. c1 .Q ..74/864, 74/866 [51] Int. Cl. ..B60k 21/00 [58] Field ofSearch ..74/864,866

[56] References Cited UNITED STATES PATENTS 3,267,762 8/1966 Reval..74/866X 3,433,101 3/1969 Scholl et a1 3,495,481 2/1970 Ohie et a1...74/864 I IO PATENTEDJAN 1 6 I975 SHEET 1 OF 3 INVENTOR NAG-AYl/(lMHRUIHD 4' NAM! IR/E PATENTEDJAN 16 I973 SHEET 2 BF 3 INVE NTOR menyu/nMHRHMO 1' NAM/0 RI:

PATENTEDJANISIQYB 3.710.651

sum 3 [1F 3 INVENTOR NAGAYHKI MARUMO 1' NAMlD IRIE LINE PRESSUREREGULATING SYSTEM FOR AUTOMATIC POWER TRANSMISSION This inventionrelates to an automatic power transmission and more particularly to asystem for regulating line pressure in a hydraulic control circuit,which pressure is transmitted to the friction-drive-establishing devicesof the transmission.

An automatic power transmission is now widely used in various motorvehicles to reduce driver effort in vehicle operation. A typical oneconsists of a torque converter, a gear train and a plurality offriction-driveestablishing devices such as friction clutches and brakeswhich, when actuated, serve to control the relative motions of theindividual working elements of the gear train in such a manner as toestablish forward and reverse drive power flows from the prime mover tothe driven member of the transmission. Actuation of the frictionclutches and brakes is effected by energizing a series of solenoidvalves through which fluid pressure, called line pressure, istransmitted to the friction clutches and brakes. It is, in thisinstance, important to have the line pressure regulated properly inaccordance with the varying driving conditions of the motor vehicle,because an excessive line pressure results in unusual mechanical shocksexperience during gear shifting and, on the contrary, an insufficientline pressure invites a slippage of brake bands which, in turn, leads toinability of starting the vehicle or attaining a desired gear ratio and,furthermore, which tends to cause premature wear of the brake bands.

In a conventional automatic transmission, however, the line pressure iskept at a sufficiently high level to prevent slippage of the brake bandswhile the problem of shift shock is left unsolvedv It has been alsotried to maintain line pressure at such a sufficiently high level atvehicle speeds below a predeterminedspeed but at a level low enough toprevent shift shock above the predetermined'speed. However, theseschemes are not satisfactory.

It is therefore an object of this invention to provide an improved linepressure regulatingsystem for an automatic power transmission with aview to overcoming the above-said disadvantages.

It is another object of thisinvention to provide a line pressureregulating system that is capable of reducing line pressure to asufficiently low level to prevent mechanical shock atthe time of gearshifting while at other times maintainingthe pressure ata level highenough to prevent slippage of the brake bands.

A better understanding of the invention may be had by referring to thefollowing description and drawings, in which:

FIG. 1 is a block diagram showing an automotive vehicle driveline and aline pressure regulating system according to this invention;

FIG. 2 is an illustration of an electronic gear shift control circuitandan electronic line pressure control circuit shown in FIG. 1;

FIG. 3 is a schematic view ofa portion of the hydraulic control deviceshown in FIG. 1, which serves to vary line pressure transmitted to thetransmission; and

FIG. 4is a circuit diagram showing another example of the electronicline pressure control circuit shown in FIG. 1.

It is intended that the line pressure regulating system of the inventionbe applicable to a large variety of automatic transmissions,particularly those in which shifting can be carried out by the operationof brakes or clutches. However, the specific embodiment herein isdesigned to apply to a transmission of the type which is described indetail in the U.S. Pat. No. 3,640,156 of Yoichi Mori et al entitledControl system for automotive automatic transmission and, therefore thedetail description of the same is herein omitted for the sake ofsimplicity of illustration.

Referring to FIG. 1, a preferred embodiment of this invention is shownin block form. In the figure, a vehicle engine 10 provided with anintake manifold 10a is connected operatively to an automatictransmission 11, the power output portion of which is connected to anoutput shaft 12. A vehicle speed sensor 13 is provided for sensingrevolutions of the output shaft 12 to generate an electric signalcorresponding to vehicle speed. The vehicle speed sensor 13 may be ofthe type which is disclosed in the U.S. Pat. No. 3,433,101 entitledElectronic arrangement for shifting gears in motor vehicles or in theU.S. Pat. No. 3,448,640 entitled Electrical control for automatictransmission. The electric signal thus generated is supplied through aline 14 to an electric gear shift control circuit 15. Another sensor 16is provided for sensing throttle opening to generate an electric signalcorresponding thereto, which signal is supplied through a line 17 to theelectronic gear shift control circuit 15. The sensor 16 may be of thetype which is disclosed in the U.S. Pat. No. 3,470,854 entitled Fuelinjection system for internal combustion engines or in the U.S. Pat. No.3,448,640 as already mentioned.

The electronic gear shift control circuit 15 functions to determineproper reduction gear ratios under different driving conditions of thevehicle in response to the electric signals from the vehicle speedsensor 13 and the throttle opening sensor 16 and, upon detection of animproper reduction gear ratio, to supply a gear shift command signal toa first electronic actuator 18. In response to the gear shift commandsignal, the first electronic actuator 18 operates a hydraulic controldevice 19in such a manner as to effect a transition to a properreduction gear ratio. The hydraulic control device 19'may be of the typewhich is disclosed in the U.S. Pat. No. 3,640,156 as already mentioned.

The gear shift command signal is also supplied to an electronic linepressure control circuit 20 through a line 21. The electronic linepressure control circuit 20 is connected through a line 22 to a secondelectronic actuator 23 such as a solenoid valve (see FIG. 3). Thefunction of the electronic line pressure control circuit 20 is to keepthe second electronic actuator 23 energized for a predetermined lengthof time. The second electronic actuator 23, when energized, serves tocooperate with the hydraulic control device 19 and an intake manifoldvacuum sensor 24 to reduce line pressure to a predetermined levelsufficiently low for preventing shift shock (which level will behereinafter referred to as dynamic level). The intake manifold vacuumsensor 24 may be of the type which is disclosed in the U.S. Pat. No.3,495,48l entitled Automatic speed change gear control device for use inautomobiles. During the time when the second electronic actuator 23 iskept de-energized the line pressure remains at another predeterminedlevel sufficiently high for preventing slippage of the brake bands ofthe friction devices (not shown). The higher predetermined pressurelevel will be hereinafter to as static level.

The predetermined length of time during which the second electronicactuator 23 is kept energized depends upon the time necessary to effecta gear shift smoothly, viz., without causing any mechanical shock. Inthe ordinary automatic transmission mounted on a motor vehicle, the timerequired is shorter than a few seconds. In the present system, thepredetermined length of time is set slightly longer than the length oftime required to effect a gear shift. By so doing, mechanical shocksexperienced in effecting a gear shift can be eliminated together withslippage of the friction devices.

Considering the fact that the present invention contemplates to reduceline pressure to the dynamic" level at the time of gear shifting whileat other times maintaining it at the static level, it is to beunderstood that the invention could equally well be applied to any typeof electronically controlled automatic transmission adapted to beresponsive to an electric command signal to effect a gear shift. Thisinvention will be described hereinafter in greater detail in connectionwith a line pressure regulating system to be mounted on an automatictransmission of the type in which a transition between two speeds offorward drive is possible in response to electric signals from athrottle opening sensor and a vehicle speed sensor.

In FIG. 2, there is shown an example of the electronic gear shiftcontrol circuit and the electronic line pressure control circuitaccording to this invention.

The electronic gear shift control circuit 15 is adapted to receive twoelectric signals V, and V from the throttle opening sensor 16 and thevehicle speed sensor 13, respectively. The electric signal V, is apositive voltage the magnitude of which increases in proportion to anincrease in throttle opening. The vehicle speed signal V is a negativevoltage the magnitude of which increases as the vehicle speed rises. Theelectric signals V, and V are supplied through resistors and 26 havingresistances R, and R respectively, to the base of a transistor 27 whichforms part of a Schmidt Trigger 28. The transistor 27 is connected atits collector to a bus line 29 of +10 volts through a resistor 30 and atits emitter to ground through a resistor 31. The collector of thetransistor 27 is also connected directly to the base of a transistor 32the emitter of which is connected to the emitter of the transistor 27.The collector of the transistor 32 is connected through a resistor 33 tothe bus line 29. In the operation of the Schmidt Trigger 28, an inputvoltages, viz., a voltage appearing at the base ofthe transistor 27changes as follows:

When V (V,/R,)xR the input voltage is negative;

and when V, (V,/R,)R it is positive.

When the input voltage becomes positive, the transistor 27 is renderedconductive to render the transistor 32 nonconductive, so that thevoltage appearing at the collector of the transistor 32 increases toapproximately +10 volts. However, while the input voltage remainsnegative because the vehicle speed is so high that the expression V(V,/R,)xR holds, an output voltage, viz., the voltage at the collectorof the transistor 32 is equal to approximately zero volts.

The output voltage is applied through a resistor 34 to the base of atransistor 35. The transistor 35 is connected at its collector to thebus line 29 through a diode 36 and at its emitter to the base of atransistor 37 the emitter of which is grounded. The collectors of thetransistors are connected together to the first electronic actuator 18to which the bus line 29 also is connected. When the output voltage ofthe Schmidt Trigger 28 increases to approximately +10 volts, thetransistor 35 is rendered conductive to render the transistor 37 alsoconductive, so that the first electronic actuator 18 is energized. Uponbeing energized, the first electronic actuator 18 operates the hydrauliccontrol device 19 in such a manner that downshift to first forward speedis effected in the automatic transmission 11. If, on the other hand, theoutput voltage of the Schmidt Trigger 28 drops to approximately zerovolts, the transistor 35 is rendered nonconductive, whereupon thetransistor 37 also is rendered nonconductive. When this occurs, thefirst electronic actuator 18 is deenergized, so that upshift to secondforward speed is effected in the automatic transmission 11. Thus, it isto be noted in this embodiment that a first forward speed is establishedwhile the output of the Schmidt Trigger 28 remains at approximately +10volts and a second forward speed is established while the output remainsat approximately zero volts. Furthermore, it is to be understood thatthe output of the Schmidt Trigger 28 is the gear shift command signal asdescribed above in conjunction with FIG. 1.

The gear shift command signal (which will be referred to as a signal Vis also supplied through a resistor 38 and a line 39 to the electronicline pressure control circuit 20, more specifically, to the base of atransistor 40 serving as an invertor. The transistor 40 has itscollector connected to the bus line 29 through a resistor 41 and itsemitter grounded. During the first forward speed, viz., when the signalvoltage V is equal to approximately 10 volts, the transistor 40 is keptconductive so that the collector voltage is approximately zero volts.During the second forward speed, the collector voltage is kept at alevel of approximately 10 volts.

Connected to the collector of the transistor 40 is a differentiator 42consisting of a capacitor 43 and a grounded resistor 44, the junctionbetween which is connected to a diode 45. The diode 45 is so polarizedas to allow only a negative pulse to pass therethrough. The negativepulse is generated by the differentiator 42 only when the signal voltageV; is changed from zero to +10 volts due to downshift from the second tofirst forward speed.

The signal V is also applied through a line 46 to another differentiator47 comprising a capacitor 48 and a grounded resistor 49, the junctionbetween which is connected to a diode 50. Like the diode 45, the diode50 is so polarized as to allow only a negative pulse to passtherethrough. Since, unlike the differentiator 42, there is no invertor40 connected to the input of the differentiator 47, the negative pulseis produced only when the signal voltage V is changed from +10 volts tozero due to upshift from the first to second forward speed. Therefore,it is to be understood that whether a gear shift effected is a downshiftor an upshift a nega-' tive pulse is allowed to pass through either thediode 45 or 50 to the following circuit.

The diodes 45 and 50 are connected together to the base of a normallyconducting transistor 51 which forms part of a monostable multivibrator52. The transistor 51 has its collector connected through a resistor 53to the bus line 29 and its emitter grounded. The collector of thetransistor 51 is also connected through a resistor 54 to the base of anormally nonconducting transistor 55, the emitter thereof beinggrounded. The collector of the normally nonconducting transistor 55 isconnected to the bus line 29 through a resistor 56 and to the base ofthe normally conducting transistor 51 through a capacitor 57. Thejunction 58 between the capacitor 57 and the base of the normallyconducting transistor 51 is connected through a resistor 59 to the busline 29. During gear shifting a negative pulse passed through the diode45 or 50 is supplied to the normally conducting transistor 51 to renderit nonconductive, triggering the monostable multivibrator 52 to itsmetastable state. The monostable multivibrator 52 is kept in themetastable state for a fixed length of time dependent upon the capacitor57 and the resistor 59. During the fixed length of time the collector ofthe normally conducting transistor 51 remains at approximately volts.The collector is connected through a resistor 60 to the base of atransistor 61. The transistor 61 is connected at its collector to thebus line 29 through a diode 62 and at its emitter to the base of atransistor 63, the emitter thereof being grounded. The collectors of thetransistors 61 and 63 are connected together to-the second electronicactuator 23 which is connected also to the bus line 29.

When the collector voltage of the transistor 51 increase toapproximately 10 volts, the transistor 61 is rendered conductive andsimultaneously the transistor 63 is rendered conductive. Thus, a currentpath is established from the bus line 29 to ground throughthe secondelectronic actuator 23-and the transistor 63. When this occurs, thesecond electronic actuator 23 acts on the hydraulic control device 19 sothat the line pressure in the hydraulic control circuit is reduced tothe dynamic" level. Upon termination of the fixed time period, themonostable multivibrator 52 resumes its original stable state tode-energize the second electronic actuator 23, so that the line pressurerises to the static" level.

In FIG. 3 there is shown diagrammatically that portion of the hydrauliccontrol device 19 which serves to vary line pressure between the staticand dynamic levels. The portion of the hydraulic control device 19 asshown comprises, essentially, an engine driven fluid pump 65, aregulator valve 66, an amplifier valve 67 and a vacuum valve 68. Theengine driven fluid pump 65 functions to deliver fluid under pressurefrom a sump 69 through a first main line 70 into a line 71 leading to anumber of friction devices (not shown) through a manual valve (notshown).

As shown, the regulator valve 66 and the amplifier valve 67 are formedintegrally with each other and include a unitary valve spool 72 which isaxially slidable and has spaced lands 73, 74, 75, 76, 77 and 78 providedthereon. A coil spring 79 is mounted in a spring pocket 80 at the lowerend of the regulator valve 66 to bias the valve spool 72 upward in thedrawing. The pressure in the line (which is the line pressure" describedabove) is transmitted through an orifice 81 to a port 82 to bias thespool 72 downward in the drawing due to the difference between thefacing areas of the lands 73 and 74 subject to pressure. The linepressure also enters the space between the lands and '76 through a port83. The port 83 is not in communication with an exhaust port 84 when thespool 72 is in the position as indicated by the right half portion ofthe spool 72. However, as the spool 72 is moved downward, the port 83 isbrought into communication with the exhaust port 84, permitting the line70 to be exhausted, so that the line pressure is reduced. The reductionin the line pressure causes a decrease in the pressure transmittedthrough the port 82 to the space between the lands 73 and 74 havingdifferent facing areas subject to pressure, so that the force tending tomove the spool 72 downward against the bias of the coil spring 79 isdiminished. This will again move the spool 72 upward. Thus, the pressurein the line 70 is automatically maintained at a certain level PL whichis determined by the biasing force F of the coil spring 79 and thedifference in facing areas S subject to pressure between the lands 73and 74 according to the following expression:

The amplifier valve 67 includes a valve housing 85 in which an axiallyslidable spool 86 viz., an extension of the regulator valve spool 72 ismounted. The amplifier valve spool 86 has lands 77 and 78 providedthereon. A port 87 formed near the lower end of the valve housing 85 isopen to the space between the lower land 78 and the bottom of the valve67. Thus, the spool 86 is biased upward by a pressure exerted on the endof the spool 86 through the port 87. This pressure is in the samedirection as that exerted on the spool 72 by the coil spring 79 toincrease line pressure.

The vacuum valve 68 includes an axially slidable spool 88 which isoperatively connected to the intake manifold vacuum sensor 24 comprisinga vacuum modulator diaphragm 89 by means of a plunger 90. The vacuummodulator diaphragm 89 is biased in a direction to move the spool 88downward by a spring (not shown) and simultaneously in a direction tomove the spool 88 upward by a vacuum transmitted through a line 91 froman intake manifold of the prime mover. Thus, as the vacuum in the intakemanifold grows higher and higher, the force tending to move the spool 88downward through the plunger is decreased. Therefore, it will beappreciated that the raised position as indicated by theright half ofthe spool 88 corresponds to a high intake manifold vacuum and thelowered position as indicated by the left half corresponds to a lowintake manifold vacuum.

The port 92 is provided in the vacuum valve 68 at a position to providecommunication to the space between lands 93 and 94 when the spool 88 isin the lowered position as shown by the left half of the spool 88. Thespool 88 has formed therein an axially extending passage 95 whichconnects the space between the lands 93 and 94 to an end chamber 96through a radial bore 95a. Thus, in the lowered position, the pressurein the line 70 transmitted through a line 97 to the port 92 enters theend chamber 96 through the bore 95a and the passage 95 and exerts aforce on the end of the spool 88, tending to force it upward in thedrawing. As shown, the vacuum valve 68 has another port 98 opening tothe space between the lands 93 and 94, the port 98 being coupled througha line 99 to a second main line 100 leading to the port 87 of theamplifier valve 67. The valve 68 also includes an exhaust port 101 whichis blocked by the land 93 when the spool 88 is in the lowered position.In the raised position, the exhaust port 101 is opened and brought intocommunication with the port 98 through the space between the lands 93and 94 to permit the line 100 to be exhausted. As a result, the pressurein the line 100 leading to the amplifier valve 67 decreases and theforce tending to move the amplifier valve spool 86 upward is reduced.Thus, it will be appreciated that the pressure in the line 70 isregulated to a level as determined by the downward force exerted on thevacuum valve spool 88 by the vacuum modulator diaphragm 89. The pressurein the line 100 is at higher levels for lower intake manifold vacuums,viz., at near full throttle and is at lower levels for higher intakemanifold vacuums, viz., at zero to light throttle.

As described above, the pressure in the line 100 is transmitted to theamplifier valve 67 and exerts a force on the end of the spool 86 to tendto move it upward in the drawing. The force thus exerted tends to closethe port 83 from exhaust to increase the pressure in the line 70. Thus,it will be understood that the pressure in the line 70 increases withthe increases of the pressure in the line 100.

A third main line 102 leading from the second main line 100 has anorifice 103 therein and is connected to a nozzle 104. Pressure in theline 102 is also transmitted through a branch line 105 to a port 106provided at the top of the regulator valve 66, and enters an end chamber107 to exert a force on the upper end of the regulator valve spool 72tending to move it downward against the biases of the coil spring 79 andthe amplifier valve 67.

The nozzle 104 is adapted to be opened and closed by the action of thesecond electronic actuator 23 which, in this embodiment, comprises aplunger 108 and a solenoid coil 109 for moving the plunger 108 to thenozzle closing position when energized. With the solenoid coil 109deenergized, the oil debouching from the nozzle 104 strikes the end ofthe plunger 108, moving it to the left. At this time, the pressure inthe line 102 is approximately at zero level. Thus, the pressure in theline 100 is transmitted only to the end chamber of the amplifier valve67, tending to move the spool 86 upward. This means that the exhaustport 84 is blocked by the land 76 with the resultant increase in thepressure in the line 70. The increased pressure in the line 70 is thestatic pressure level described above.

When the second electronic actuator 23 is energized in response to theelectric signal from the electronic line pressure control circuit 20,the plunger 108 is moved to the nozzle closing position. With the nozzle104 closed, the pressure in the line 102 rises to a level which isapproximately equal to that in the line 100. Thus, the regulator valvespool 72 is biased downward by a pressure transmitted through the branchline 105 to the end chamber 107. This spool movement will tend to openthe exhaust port 84, as a result, reduce the pressure in the line 70.The lower pressure in the line which is maintained in the entire systemwhile the electronic second actuator 23 is energized is the dynamic linepressure described above.

FIG. 4 is a circuit diagram showing another example of the presentelectronic line pressure control circuit 20 which is different from thatof FIG. 2 in that the time during which the line pressure is maintainedat the dynamic level varies with vehicle speed. In the figure, likeparts and components of the circuit are designated by the same referencenumerals as used in FIG. 2. A portion of the circuit enclosed within arectangle 110 of broken lines corresponds to the resistor 59 shown inFIG. 2. A resistor 111 is connected between the bus line 29 and the baseof the nonnally conducting transistor 51. Another resistor 112 is alsoconnected at one end to the base of the transistor 51 and at the otherend to the emitter of a transistor 113. The transistor 113 is connectedat its collector to the bus line 29 and at its base to a resistor 114leading to the bus line 29. The output signal V of the vehicle speedsensor 13 is applied to the base of the transistor 113 to modulate theconductivity of the collector-emitter path. Since, as described above,the signal V is a negative voltage the magnitude of which increases asthe vehicle speed rises, the conductivity of the collector-emitter pathdecreases with increasing vehicle speed. Thus, with the increase ofvehicle speed, the equivalent resistance given by the resistors 111 and112 and the transistor I13 increases with the resultant increase in thetime T during which the line pressure is kept at the dynamic" level. Onthe other hand, when the vehicle speed drops, the time T decreases.According to this invention, the time T is set slightly longer than thetime required to effect a gear shift at different vehicle speeds, forthe purpose of preventing the friction devices from being criticallydamaged. The reason is that upon termination of the time T the linepressure resumes its original static level preventing the slippage ofthe cooperating friction devices.

In this embodiment, it is to be understood that the time T can berelated also to engine output torque. In this instance, the outputtorque signal to be applied to the base of the transistor 113 should bea negative voltage the magnitude of which increases as the output torquerises. With this arrangement, the time T during which the line pressureis kept at the dynamic" level varies with vehicle speed and engineoutput torque, insuring a perfect and shock-free gear shift.

What is claimed is:

l. A system for regulating a line pressure in a hydraulic controlcircuit for an automatic power transmission, for a motor vehicle havingan internal combustion engine provided with an intake manifold and athrottle valve, having a plurality of friction-drive-establishingdevices, comprising: a first means for producing a first electric signalcorresponding to vehicle speed, a second means for producing a secondelectric signal corresponding to a throttle opening, a first circuitmeans for determining proper reduction gear ratios under differentdriving conditions of the motor vehicle in response to said first andsecond electric signals to produce a gear shift command signal, saidfirst circuit means consisting of a Schmidt Trigger circuit having acommon input connected to said first and second means, a firstelectronic actuator responsive to said gear shift command signal foroperating the hydraulic control circuit so as to effect a transition toa proper reduction gear ratio, a second circuit means for producing aline pressure reduction signal for a predetermined length of time inresponse to said gear shift command signal, said second circuit meansconsisting of means for producing a trigger pulse in response to saidgear shift command signal and means for producing said line pressurereduction signal in response to said trigger pulse, a second electronicactuator responsive to said line pressure reduction signal, and ahydraulic control means for reducing said line pressure to apredetermined level sufficiently low by means of said second electronicactuator in order to prevent shift shock, said hydraulic control meansincluding a means for sensing a vacuum in the intake manifold and saidline pressure being normally maintained at a sufficiently high level inorder to prevent slippage of the friction-drive-establishing devices.

2. A system according to claim 1, in which said means for producing atrigger pulse comprises a series combination of an invertor, a firstdifferentiator and a first diode and a series combination of a seconddifferentiator and a second diode, and said means for producing saidline pressure reduction signal in response to said trigger pulsecomprises a monostable multivibrator adapted to be kept in metastablestate, said two series combinations being connected in parallel with theinput of said monostable multivibrator and the length of time duringwhich said monostable multivibrator is kept in its metastable statedetermining the duration of said line pressure reduction signal.

3. A system according to claim 2, in which said length of time increasesas said vehicle speed rises and further increases in proportion to anincrease in said throttle opening.

4. A system according to claim 1, in which said second electronicactuator comprises a solenoid coil connected to said second circuitmeans and a plunger 5. A system according to claim 1, in which saidhydraulic control means comprises a first main line leading from theoutlet port of an engine driven fluid pump, a regulator valve connectedto said first main line and having an axially slidable spool therein,said spool being slidable in one direction to increase the pressure insaid first main line and in the opposite direction to reduce thepressure in said first main line tending to slide said spool in theopposite direction, a vacuum valve connected through a branch line tosaid first main line and having an axially slidable spool therein, saidspool being operatively connected to the intake manifold vacuum sensorin such a manner that as the intake manifold vacuum increases said spoolslides in one direction and as the intake manifold vacuum decreases saidspool slides in the opposite direction, a second main line connected tosaid vacuum valve in such a manner that when said spool slides in onedirection the pressure in said second line decreases and when said spoolslides in the opposite direction the pressure increases an amplifiervalve having an axially slidable spool and connected to said second mainline in such a manner that the ressure in said second main hne exerts aforce on sai spool tending to slide it in one direction, said spoolbeing operatively connected to said regulator valve spool in such amanner that when said amplifier valve spool slides in one direction saidregulator valve spool slides also in said one direction, a third lineconnected to said second line through an orifice, saidthird main linebeing connected to said regulator valve in such a manner that thepressure in said third main line tends to cause the regulator valvespool to slide in said opposite direction, a nozzle connected to saidthird main line, said nozzle being operatively closed by means of saidplunger of the second electronic actuator in response to said linepressure reduction signal, whereby when said solenoid coil is energizedby said line pressure reduction signal, the pressure in said third mainline increases so as to cause said regulator valve spool to slide insaid opposite direction and reduce the pressure in said first main line.

1. A system for regulating a line pressure in a hydraulic control circuit for an automatic power transmission, for a motor vehicle having an internal combustion engine provided with an intake manifold and a throttle valve, having a plurality of friction-drive-establishing devices, comprising: a first means for producing a first electric signal corresponding to vehicle speed, a second means for producing a second electric signal corresponding to a throttle opening, a first circuit means for determining proper reduction gear ratios under different driving conditions of the motor vehicle in response to said first and second electric signals to produce a gear shift command signal, said first circuit means consisting of a Schmidt Trigger circuit having a common input connected to said first and second means, a first electronic actuator responsive to said gear shift command signal for operating the hydraulic control circuit so as to effect a transition to a proper reduction gear ratio, a second circuit means for producing a line pressure reduction signal for a predetermined length of time in response to said gear shift command signal, said second circuit means consisting of means for producing a trigger pulse in response to said gear shift command signal and means for producing said line pressure reduction signal in response to said trigger pulse, a second electronic actuator responsive to said line pressure reduction signal, and a hydraulic control means for reducing said line pressure to a predetermined level sufficiently low by means of said second electronic actuator in order to prevent shift shock, said hydraulic control means including a means for sensing a vacuum in the intake manifold and said line pressure being normally maintained at a sufficiently high level in order to prevent slippage of the friction-drive-establishing devices.
 2. A system according to claim 1, in which said means for producing a trigger pulse comprises a series combination of an invertor, a first differentiator and a first diode and a series combination of a second differentiator and a second diode, and said means for producing said line pressure reduction signal in response to said trigger pulse comprises a monostable multivibrator adapted to be kept in metastable state, said two series combinations being connected in parallel with the input of said monostable multivibrator and the length of time during which said monostable multivibrator is kept in its metastable state determining the duration of said line pressure reduction signal.
 3. A system according to claim 2, in which said length of time increases as said vehicle speed rises and further increases in proportion to an increase in said throttle opening.
 4. A system according to claim 1, in which said second electronic actuator comprises a solenoid coil connected to said second circuit means and a plunger.
 5. A system according to claim 1, in which said hydraulic control means comprises a first main line leading from the outlet port of an engine driven fluid pump, a regulator valve connected to said first main line and having an axially slidable spool therein, said spool being slidable in one direction to increase the pressure in said first main line and in the opposite direction to reduce the pressure in said first main line tending to slide said spool in the opposite direction, a vacuum valve connected through a branch line to said first main line and having an axially slidable spool therein, said spool being operatively connected to the intake manifold vacuum seNsor in such a manner that as the intake manifold vacuum increases said spool slides in one direction and as the intake manifold vacuum decreases said spool slides in the opposite direction, a second main line connected to said vacuum valve in such a manner that when said spool slides in one direction the pressure in said second line decreases and when said spool slides in the opposite direction the pressure increases an amplifier valve having an axially slidable spool and connected to said second main line in such a manner that the pressure in said second main line exerts a force on said spool tending to slide it in one direction, said spool being operatively connected to said regulator valve spool in such a manner that when said amplifier valve spool slides in one direction said regulator valve spool slides also in said one direction, a third line connected to said second line through an orifice, said third main line being connected to said regulator valve in such a manner that the pressure in said third main line tends to cause the regulator valve spool to slide in said opposite direction, a nozzle connected to said third main line, said nozzle being operatively closed by means of said plunger of the second electronic actuator in response to said line pressure reduction signal, whereby when said solenoid coil is energized by said line pressure reduction signal, the pressure in said third main line increases so as to cause said regulator valve spool to slide in said opposite direction and reduce the pressure in said first main line. 